Multi-Cycle CPU
Multi-Cycle CPU Combine Functional Units Reuse for different phases of instructions One ALU for PC increment Branch target computation Address computation for memory access R-Type instruction execution One memory unit for both instructions and data Multiple but Shorter Clock Cycles Different instructions take different number of cycles Average CPI times cycle time gives better performance
F A D A B B E C PCWrite Mem Read Mem Write IRWrite RegWrite ALUOp PC To Control 31-21 0 M 1 U X PCWrite F Zero Mem Read Mem Write PC A D IRWrite RegWrite 0 M U 1 X 0 M U 1 X 5-9 Read/Write Addr IR Read Reg 1 Read Data 1 A 16-20 0 M U 1 X B A L U ALU Out Memory Read Reg 2 Registers 0 M 1 U 2 X 3 Read Data 2 B Mem Data Write Reg 4 Write Data 0-4 Write Data E C 0 M U 1 X Mem Data Reg Sign Ext Shift Left 2 0-31 ALU Ctrl 21-31 ALUOp
Instruction Fetch/PC Increment To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr IR Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 ALU Out Memory B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg Sign Ext Shift Left 2 15-0 ALU Ctrl 5-0 ALUOp ADD Instruction Fetch/PC Increment
Instruction Decode/Register Fetch/Branch Target To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 ALU Out Memory IR B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Instruction Decode/Register Fetch/Branch Target
Branch Completion (Branch Taken) PCWriteCond To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp SUB Branch Completion (Branch Taken)
Instruction Fetch after Branch Completion To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Instruction Fetch after Branch Completion
Instruction Decode/Register Fetch/Branch Target To Control 31-26 1 PCWrite PC 31-28 F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Instruction Decode/Register Fetch/Branch Target
R-Type Instruction Execution To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp Function R-Type Instruction Execution
R-Type Instruction Write Back To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp R-Type Instruction Write Back
Instruction Fetch After R-Type To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Instruction Fetch After R-Type
Instruction Decode/Register Fetch/Branch Target To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Instruction Decode/Register Fetch/Branch Target
Address Computation (lw or sw) To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Address Computation (lw or sw)
Memory Access for lw F A D A B B C E PCWrite Mem Read Mem Write To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp Memory Access for lw
Write Back for lw F A D A B B C E PCWrite Mem Read Mem Write IRWrite To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp Write Back for lw
Instruction Fetch after lw To Control 31-26 1 PCWrite F Zero Mem Read Mem Write A D PC IRWrite RegWrite 1 1 25-21 Read/Write Addr Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 Memory IR ALU Out B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg 15-0 Shift Left 2 Sign Ext ALU Ctrl 5-0 ALUOp ADD Instruction Fetch after lw
F A D A B B C E PCWrite Mem Read Mem Write IRWrite RegWrite ALUOp PC To Control 31-26 1 PCWrite F Zero Mem Read Mem Write PC A D IRWrite RegWrite 1 1 25-21 Read/Write Addr IR Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 ALU Out Memory B Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data C 1 E Mem Data Reg Sign Ext Shift Left 2 15-0 ALU Ctrl 5-0 ALUOp
Control A B ALUOp PCWriteCond PCSrc PCWrite MemWrite ALUSrcA MemRead 31-26 1 PCWrite Zero MemWrite ALUSrcA MemRead PC IorD IRWrite RegDst RegWrite 1 1 25-21 Read/Write Addr IR Read Reg 1 Read Data 1 A 20-16 A L U Read Reg 2 ALU Out Memory Registers 1 Read Data 2 B 1 2 3 Mem Data Write Reg 4 Write Data 15-11 Write Data 1 ALUSrcB Mem Data Reg Sign Ext Shift Left 2 15-0 ALU Ctrl MemtoReg 5-0 ALUOp
Control Lines PCWriteCond Write PC conditionally on branch PCWrite Write PC for increment or jump PCSrc Select source for writing to PC IorD Select address for memory read/write MemRead Read from memory (instruction or data) MemWrite Write to memory (store word) IRWrite Write to Instruction Register
Control Lines MemtoReg Select memory or ALUOut to write to register RegDst Select field to select destination register RegWrite Write to selected register ALUSrcA Select source for upper ALU input ALUSrcB Select source for lower ALU input ALUOp Select ALU operation or set to function code: 00 = Add, O1 = Subtract, 10 = use funct field (bits 0-5)
Clock Cycles 1. Instruction Fetch, PC Increment 2. Instruction Decode, Register Fetch, Branch Target Computation 3. R-type Execution or Memory Address Computation or Branch Completion 4. R-type Write Back or Memory Access 5. Memory write back
Control Overview Instruction Fetch Instruction Decode Start Memory Access R-type Cond Branch Uncond Branch
Control: Finite State Machine Instruction Fetch 0 Instruction Decode 1 MemRead IorD = 0 IRWrite ALUSrcA = 0 ALUSrcB = 01 ALUOp = 00 PCSrc = 00 PCWrite ALUSrcA = 0 ALUSrcB = 11 ALUOp = 00 Start ldur or stur R-type cbz b Memory Access FSM Cond Branch FSM Branch FSM R-type FSM
Memory Access FSM Memory Address Comp 2 From State 1 lw or sw Memory ALUSrcA = 1 ALUSrcB = 10 ALUOp = 00 Memory Access lw 3 ldur stur MemRead IorD = 1 MemWrite IorD = 1 Memory Access sw 5 Write Back 4 RegWrite MemToReg = 1 RegDst = 0 To State 0
R-type FSM From State 1 R-type Execution 6 Write Back 7 To State 0 ALUSrcA = 1 ALUSrcB = 00 ALUOp = 10 Execution 6 RegDst = 1 RegWrite MemtoReg = 0 Write Back 7 To State 0
Conditional Branch FSM From State 1 cbz ALUSrcA = 1 ALUSrcB = 00 ALUOp = 01 PCWriteCond PCSrc = 1 Branch Completion 8 To State 0
(Unconditional) Branch FSM From State 1 cbz ALUSrcA = 1 ALUSrcB = 00 ALUOp = 01 PCWrite PCSrc = 1 Branch Completion 8 To State 0
Complete Finite State Machine Instruction Fetch 0 MemRead IorD = 0 IRWrite ALUSrcA = 0 ALUSrcB = 01 ALUOp = 00 PCSrc = 00 PCWrite Instruction Decode 1 ALUSrcA = 0 ALUSrcB = 11 ALUOp = 00 Start ldur or stur R-type b cbz ALUSrcA = 1 ALUSrcB = 10 ALUOp = 00 Unconditional Branch Completion 9 Memory Address Comp 2 ALUSrcA = 1 ALUSrcB = 00 ALUOp = 10 Execution 6 ALUSrcA = 1 ALUSrcB = 00 ALUOp = 01 PCWrite PCSrc = 1 ldur stur Memory Access ldur 3 MemRead IorD = 1 Conditional Branch Completion 8 ALUSrcA = 1 ALUSrcB = 00 ALUOp = 01 PCWriteCond PCSrc = 1 MemWrite IorD = 1 RegDst = 1 RegWrite MemtoReg = 0 Memory Access stur 5 Write Back 7 RegWrite MemToReg = 1 RegDst = 0 Write Back 4
FSM Implementation as PLA Opcode from IR PLA Clock State Register PCWrite ALUOp PCWriteCond ALUSrcB PCSrc IorD ALUSrcA MemRead RegWrite MemWrite RegDst IRWrite MemtoReg